1. Field of the Art
The present invention relates to a process for preparing N-methylated melamines characterized by heating melamine (2,4,6-triamino-1,3,5-triazine) with methylamine (monomethylamine) in the presence of an acid catalyst under pressure to substitute at least one amino group of the melamine by methylamino group(s).
Specifically, the reaction of melamine and methylamine in the presence of an acid catalyst leads N-methylated melamines including 2,4-diamino-6-methylamino-1,3,5-triazine (mono-type), 2-amino-4,6-bis(methylamino)-1,3,5-triazine (bis-type) and 2,4,6-tris(methylamino)-1,3,5triazine (tris-type) as desired compounds.
N-methylated melamines are useful compound groups that are used widely as various fine chemical intermedIates for agricultural chemicals, medicines, dyes, paints and the like, as forming components for various resin materials, in particular an aminoplast, and as a flame retardant.
2. Description of the Related Art
As methods for preparing N-alkylated melamines, synthetic methods by reacting cyanuric chloride with an alkylamine are reported (J. Amer. Chem. Soc., 73,2984 (1951) and U.S. Pat. Nos. 5,124,379 and 4,886,882). The synthetic methods require expensive cyanuric chloride, a plurality steps for synthesizing mono-type and bis-type of N-alkylated melamines and further disposal of hydrochloric acid in post-treatment.
Further, a synthetic method disclosed in U.S. Pat. No. 2,228,161 (corresponding to GB Patent No. 496,690) is also known. The synthetic method comprises reacting melamine with hydrochloride of an alkylamine. As the reaction requires to be carried out in a state of solid by using hydrochloric acid, an amount of reactants to be treated is very large. In addition, the reaction needs countermeasures against corrosion in an apparatus and lowering in quality of products due to the use of the hydrochloride in a high temperature. Further, the reaction necessitates disposal of hydrochloric acid in post-treatment. The above-mentioned U.S. Pat. No. 2,228,161 discloses a method as a synthetic method of N-methylated melamines in which melamine and methylamine hydrochloride are reacted at 200° C. on a solid phase. The patent publication reports that the yield of 2-amino-4,6-bis(methylamino)-1,3,5-triazine (bis-type) is 50 to 55% N-methylated melamines and that a small amount of 2,4,6-tris(methylamino)-1,3,5-triazine (tris-type) is obtained.
It is known that the yield by a method in which methylol melamine is reduced is low and that the method requires treatment of polymers that are generated as by-product